This invention relates to an apparatus for isotope exchange reaction, and particularly to an apparatus for isotope exchange reaction with a large treating capacity per unit volume of catalyst.
Heavy water-moderated nuclear reactors use heavy water as a moderator, and tritium (T) is liable to be generated by action of neutron [(n, .gamma.) reaction]. Tritium is a radioactive isotope of hydrogen, and its behavior is substantially similar to that of hydrogen. It exists in the state of tritium oxide (tritium water) in heavy water. An increase in the proportion of tritium water gives rise to a risk that operators will absorb tritium into their bodies when heavy water containing tritium water is leaked from the system of heavy water-moderated nuclear reactor or when a heavy water-moderated nuclear reactor is subjected to maintenance and inspection, particularly at replacement work of system pipings. In order to prevent such a risk, an apparatus for removing tritium is provided in the heavy water-moderated nuclear reactor.
As general process for separating and recovering deuterium (D) and tritium (T) as hydrogen isotopes contained in a small amount in light water (H.sub.2 O) and tritium (T) contained in a small amount in heavy water (D.sub.2 O), there has been proposed a process utilizing an isotope exchange reaction between liquid water and hydrogen gas (Japanese Patent Publication No. 18680/74). The isotope exchange reaction between liquid water and hydrogen gas has a very low reaction rate, and thus it is necessary to use a hydrophobic catalyst having a catalyst surface of good contactibility with heavy water and deuterium gas to promote the reaction.
As an apparatus for isotope exchange reaction, there have been already proposed a water spray-type reactor column for isotope exchange reaction (which will be hereinafter referred to as "water spray-type reactor column") and a hydrogen gas bubbling-type reactor column for isotope exchange reaction (which will be hereinafter referred to as "hydrogen gas-bubbling type reactor column").
Separation and recovery of tritium in the water spray-type reactor column are carried out in the following manner.
Heavy water containing tritium water (DTO) is supplied to a hydrophobic catalyst bed in a reactor column from the top of the catalyst bed and undergoes countercurrent gas-liquid contact with hydrogen gas supplied into the hydrophobic catalyst bed from the bottom of the catalyst bed on the surface of the hydrophobic catalyst. At that time, the following isotope exchange reaction takes place: EQU DTO (liquid)+D.sub.2 (gas).revreaction.D.sub.2 O (liquid)+DT (gas) (1)
When the tritium concentration of deuterium gas is sufficiently low, the reaction of equation (1) proceeds to the right side from the left side, and as a result tritium is separated from the heavy water containing tritium water.
The water spray-type reactor column is to improve the dispersibility of heavy water when supplied into the hydrophobic catalyst bed and enhance the reaction efficiency of isotope exchange reaction. Since the catalyst is hydrophobic, the heavy water supplied in a sufficiently dispersed state to the hydrophobic catalyst bed are formed into spherical water droplets in the hydrophobic catalyst bed according to its surface tension. The water droplets further undergo coagulation in the hydrophobic catalyst bed and gradually grow into larger water droplets. Thus, the flow of heavy water through the catalyst bed becomes very uneven, considerably deteriorating the gas-liquid contact efficiency on the surfaces of hydrophobic catalyst and lowering the efficiency of isotope exchange reaction shown by equation (1).
The hydrogen bubbling-type reactor column is disclosed in Japanese Laid-open Patent Applications Nos. 54696/78 and 54697/78, and is to improve the gas-liquid contact efficiency and enhance the reaction rate by conversion of heavy water in vapor in a gas state and reaction of the vapor with deuterium gas. That is, the deuterium gas flows through the reactor column from the bottom upwards whereas the heavy water flows down through the overflow conduit provided in the reactor column, flows over a perforated tray in the reactor column in a direction perpendicular to the flow direction of deuterium gas and falls onto another lower perforated tray through a one-stage lower overflow conduit. The deuterium gas is saturated with the water vapor when passed through the water accumulated on the perforated trays, becomes a mixture of deuterium and heavy water vapor and passes through the hydrophobic catalyst bed in the reactor column. At that time, the following hydrogen isotope exchange reaction takes place between the vapor of heavy water and deuterium. EQU DTO (gas)+D.sub.2 (gas).revreaction.D.sub.2 O (gas)+DT (gas) (2)
Tritium in heavy water is transferred into deuterium gas thereby, and tritium in heavy water is removed.
In the hydrogen gas bubbling-type reactor column, a gas-liquid contact reaction between the vapor of heavy water and deuterium takes place in the catalyst bed, and thus the contact reaction between the heavy water in a gas state and deuterium gas can be made even, and the reaction efficiency of isotope exchange reaction can be considerably elevated. That is, tritium can be removed from heavy water with a good efficiency correspondingly. However, the capacity to treat the heavy water in the hydrogen gas bubbling-type reactor column depends upon the amount of saturating vapor in the deuterium gas, and consequently the hydrogen gas bubbling-type reactor column has such a disadvantage as a small capacity to treat heavy water per unit volume of the hydrophobic catalyst.